1. Field of the Invention
The invention relates to systems for detecting temperature in combustion turbines upstream of the combustion zone with optical fiber Bragg temperature sensors, and more particularly to systems employing such temperature sensors for detecting combustion turbine flashback.
2. Description of the Prior Art
Two objectives in design and operation of gas turbine combustors are the stability of the flame and prevention of flashbacks. A flashback occurs when flame travels upstream from the combustion zone in the combustion chamber and approaches, contacts, and/or attaches to, an upstream component, such as the venturi or premix chamber. Or, the flame may flash back into a fuel/air mixing apparatus, damaging components that mix the fuel with the air. Upstream components are not designed to withstand combustion temperatures for significant time exposure before they are damaged and need repair.
A multitude of factors and operating conditions provide for efficient and clean operation of the gas turbine combustor area during ongoing operation. Although a stable lean mixture is desired for fuel efficiency and for environmentally acceptable emissions, a flashback may occur more frequently with a lean mixture, and particularly during an unstable operating condition of the engine. Not only is the fuel/air mixture important, also relevant to gas turbine operation are the shape of the combustion area, the arrangement of assemblies that provide fuel, and the length of the combustor that provides varying degrees of mixing. Given the efficiency and emissions criteria, the operation of gas turbines requires a balancing of design and operational approaches to maintain efficiency, meet emission standards, and avoid damage due to undesired flashback occurrences.
Flashback conditions are monitored with flashback detectors so that corrective action may be taken to avoid the above-described damage to upstream combustor components. Known flashback detectors utilize thermocouple sensors mounted within the combustor basket assemblies and feed sensor readings to a temperature measurement apparatus. Flashback thermocouple sensors are generally designed to have a response time of less than three seconds, so that timely corrective actions can be performed to abate a flashback event. Temperature information is utilized by a flashback detection system that associates changes in absolute temperature with a flashback condition. Upon associating a temperature change with a flashback condition the information is routed to the turbine combustor fuel/air control system for remediation in accordance with known control parameters (e.g., increase combustor intake airflow to suppress or extinguish the flashback flame front).
Thermocouples are individually hard-wire connected to the temperature measurement apparatus. Thermocouple sensor lead wires are routed in the combustor basket in bundles. Given wiring complexities and limited space confines within the combustor basket envelope often one—or no more than a few—thermocouples are installed in each combustor basket. Thermocouples are exposed to the harsh, relatively hot environment within the combustor basket and susceptible to damage during flashback events. It is desirable to increase the number of temperature sensing and interrogation points within individual combustor baskets, to get more detailed information about the severity of an incipient flashback event, and preferably obtain more response lead time to take remedial action before the combustor basket suffers thermal damage. It is also desirable to utilize sensors that have quicker transient temperature response time, that are robust in construction for heat damage tolerance and relatively simpler to install than known thermocouples.
Commercially available fiber optic temperature sensors employing fiber Bragg grating (FBG) sensors are known for robust, heat resistant construction, capable of withstanding temperatures in the 1800° F. (˜1000° C.) range. FBG sensors cause a reflected output wavelength that is associated with sensor temperature. A string of serially coupled FBG sensors on a single fiber optic strand are capable of transmitting temperature information (reflected wavelength) from each sensor to a known optical interrogator. The interrogator or a temperature measurement device coupled to the interrogator associates the received reflected wavelengths with sensor temperature. Hence, a single fiber optic sensing cable, coupled to a light source and an optical interrogator can transmit multiple, essentially simultaneous, temperature information reflected waveforms back to the interrogator, which has quick sampling rates and quick sampling speeds.
However, commercial fiber optic FBG temperature sensor cables are not suitably packaged for insertion into gas turbine combustor baskets under continuous operating conditions. Among other things, the naked FBG sensor cables need to be protected from combustion flashback, entrained fuel/airflow erosion, and/or vibration damage. Naked FBG sensor cables also need to be isolated from influences in the combustor basket environment that might impact accurate temperature and/or temperature rate of change readings. Strain-induced wavelength shifts may negatively influence temperature-induced wavelength shifts.
FBG sensors wrapped in protective insulative fabric and encased with heavy metal shielding have been proposed for measuring gas turbine exhaust temperatures and for obtaining temperature profiles within turbine exhaust streams. Some embodiments of exhaust temperature profile sensors include window cutouts proximal FBG sensors. While exhaust temperature profile FBG sensors have sufficient external shielding and wrapped insulation to survive in the very hot combustion environment, their shielding/insulation thermal mass (and low thermal conductivity) is not satisfactory for obtaining accurate, rapidly changing temperature information and temperature rate of change information that is desirable for combustor basket flashback detection upstream of the turbine combustion zone.